The goal of the proposed research is to characterize the frequency, properties, and mechanisms of gene conversion in mice. Gene conversion is the non-reciprocal exchange of genetic information. It is a product of fundamental recombination mechanisms. The era of molecular biology has dramatically illustrated the role of gene duplication in genome evolution. Duplication followed by divergence and selective adaptation is the principle way that new genes are created. Unequal recombination between homologous chromosomes or sister chromatids creates locally dispersed families of genes with related but specialized functions. Duplicated genes are susceptible to gene conversion, which has the potential to create new alleles in single genetic events. It can increase or decrease functional diversity, depending on the sequences involved. Gene conversion can be extremely frequent in fungi. Results in the first grant period demonstrated that the same is true in mice. These observations present paradoxes that are addressed in this proposal : How does the genome remain relatively stable if subject to potentially active forms of illegitimate recombination? The proposed experiments exploit a strategy that circumvents the two major technical and logistical obstacles that previously limited investigations of mammalian gene conversion : generating large numbers of progeny and detection of conversions. This system involves scoring of transgenic mouse spermatids for contrived recombination events. The occurrence of a planned event results in correction of a mutated reporter gene (LacZ) whose activity is readily detectable in spermatids. Intrachromosomal and ectopic conversion was observed to occur at high frequencies of up to 2%. These studies have enabled a new series of critical questions regarding gene conversion to be addressed: 1) How does template length and sequence heterology affect gene conversion frequency? 2) What genetic and epigenetic factors modulate conversion? 3) Is transgene recombination reflective of endogenous genes? 4) Does meiotic recombination in mammals occur by mechanisms similar to lower organisms? These experiments will be instrumental for assessing the evolutionary and functional ramifications of gene conversion upon the genome. Additionally, the insights into mechanisms of recombination will increase comprehension of deleterious genome rearrangements, and assist attempts to manipulate the genome in productive ways.